The present invention relates generally to gas turbine engines, and, more specifically, to milling of blisks therein.
A turbofan gas turbine engine includes in serial flow communication a fan through which ambient air is propelled, a multistage compressor for pressurizing a portion of the air, a combustor wherein the compressed air is mixed with fuel and ignited for generating hot combustion gases, and high and lower pressure turbines which extract energy from the combustion gases for powering the compressor and fan, respectively. Both the fan and compressor include airfoils in the form of rotor blades which extend radially outwardly from a rotor disk. The blades are freestanding or cantilevered radially outwardly from the disk and the centrifugal loads generated therein during operation must be carried by the disk within acceptable stress limits.
A typical rotor blade is carried in the disk by an integral dovetail which slides into a corresponding dovetail slot in the perimeter of the disk. However, as the number of rotor blades around the perimeter of the disk increases in some designs, insufficient material is available around the perimeter of the disk for supporting the several blades within the acceptable stress limits.
Accordingly, blisks have been developed and are found in commercial use and are characterized by the absence of independent dovetails, with the blades instead being integrally joined to the rotor disk in a one-piece, unitary assembly. In this way, fan or compressor blisks may be used in an engine for maximizing aerodynamic efficiency thereof while reducing the associated centrifugal stresses in the supporting disk.
However, the manufacture of a blisk is substantially more complex than the manufacture of individual rotor blades and discrete rotor disks, and manufacturing defects therein involve a substantially greater risk. If even one of the several rotor blades being manufactured is outside an acceptable manufacturing tolerance, the entire blisk is defective and must be scrapped at considerable expense. Accordingly, the manufacture of a single blisk requires great care and attention to manufacturing tolerances which increases the time to manufacture the blisk, and correspondingly increases the cost thereof.
For example, one conventional method of manufacturing a blisk includes a multistep point machining or milling individual blades therein in turn. Since the individual blades have airfoils specifically configured for their fan or compressor functions, the blades typically twist about a radial axis from their roots to tips, have varying taper, and have generally concave pressure sides with correspondingly opposite generally convex suction sides. The complex three-dimensional shape of the individual blades is precisely point milled with a suitable cutter or cutting tool in the form of a ball end mill. This mill includes a semispherical cutting end attached to a cylindrical shaft which rotates the mill for removing material in grooves from the blank in multiple passes.
The point milling process typically includes a first step of rough milling individual pockets axially across the perimeter of the blank between its opposite axial faces in successive steps or levels until the pocket has a substantially complete depth. The blank is indexed to repeat the rough milling of the several pockets around the perimeter of the blank with the remaining material between the blanks being freestanding, radially outwardly extending cantilevers which roughly approximate the 3-D shape of the individual airfoils with a suitable excess of material therearound. This rough milling uses a relatively large ball end mill which is plunged into the blank in each pass over substantially its full radius to maximize material removal.
A second step in the process uses a smaller ball end mill to semi-finish the platform at the bottom of each pocket and provide suitably small radii with the freestanding rough airfoils.
A third step uses yet another ball end mill of the initially large diameter to semi-finish the pressure and suction sides of the individual rough airfoils in turn. The end mill typically circles each rough airfoil in multiple cutting steps from the outer tip to the inner root near the platform, and uses suitably smaller tolerances to produce a semi-finish airfoil still having excess material.
A fourth step uses yet another, small ball end mill to finish machine the individual platforms at the bottom of the pockets to final dimensions with suitably small tolerances.
And, the last step in machining the airfoil blades uses yet another large ball end mill to finish machine the individual airfoils in turn by again circling the individual airfoils from tip to root in multiple steps or passes with suitably small tolerances to achieve the precise dimensions of the required airfoils.
In another conventional manufacturing process, the individual airfoils may be rough and semi-finish milled as in the above process, with the final machining thereof being effected using electrochemical machining (ECM). ECM machining is precise and relatively quick, yet requires expensive equipment which correspondingly increases the cost of manufacture.
In both methods, however, the individual rough airfoils must nevertheless be semi-finish milled with a ball end mill which necessarily applies a contact force on the airfoils. Since the airfoils are freestanding or radially cantilevered, they inherently elastically deflect under the force of the cutting mill, which must be accommodated in the milling process to avoid removing excessive material which would render the blisk defective. Airfoil flexibility is accommodated by removing relatively little material in each pass to minimize the elastic deflection of the airfoils, at a considerable increase in milling time. Since ball end milling cuts a series of grooves along the surface of the airfoil it leaves intervening cusps which must be minimized in height for achieving an acceptably smooth final surface contour of the pressure and suction sides of the individual airfoils.
Furthermore, the individual blisk airfoils are individually milled and therefore additional manufacturing variations occur statistically from blade-to-blade. Since the resulting blisk is a rotor component which operates at substantial rotary speed, it must be suitably dynamically balanced during operation. Balancing is typically effected by providing an annular balancing land near the hub of the disk from which material may be precisely machined for balancing the entire blisk.
However, the balance correction effected at the land necessarily has a limit which sometimes may be insufficient in the event of excess variation in final dimensions of the several blisk airfoils. In order to balance such a blisk, individual airfoils may require additional milling provided sufficient material remains thereon in order to reduce the initial unbalanced condition of the blisk.
Accordingly, it is desired to provide an improved process for milling gas turbine engine blisks with improved efficiency and accuracy.